Semiconductor device

ABSTRACT

The invention enhances resistance to a surge in a semiconductor device having a semiconductor die mounted on a lead frame. An N type embedded layer, an epitaxial layer and a P type semiconductor layer are disposed on the front surface of a P type semiconductor substrate forming an IC die. A metal thin film is disposed on the back surface of the semiconductor substrate, and a conductive paste containing silver particles and so on is disposed between the metal thin film and a metal island. When a surge is applied to a pad electrode disposed on the front surface of the semiconductor layer, the surge current flowing from the semiconductor layer into the semiconductor substrate runs toward the metal island through the metal thin film.

CROSS-REFERENCE OF THE INVENTION

This application claims priority from Japanese Patent Application No.2010-208814, the content of which is incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a semiconductor device, in particular, asemiconductor device having a semiconductor die mounted on a lead frame.

2. Description of the Related Art

In a semiconductor device having a plurality of semiconductor dies suchas an IC die, as shown in FIG. 6, for example, an IC die 110A is bondedto an island 150 of a lead frame made of metal such as copper through aconductive paste 140.

A semiconductor device having a semiconductor die mounted on a leadframe is disclosed in Japanese Patent Application Publication Nos.2010-80914 and 2006-32479, for example.

However, depending on the purpose of use of a semiconductor device, asshown in FIG. 6, a surge (a pulse-shaped overvoltage having a highleading edge of an amplitude) tends to be applied thereto through a padelectrode 111 disposed on the front surface of the IC die 110A. Forexample, when the semiconductor device is an igniter controlling anignition plug on a vehicle, a large surge tends to be applied theretodue to noise occurring at other peripheral device on the vehicle such asa motor.

Depending on the amount of a surge, a surge current flowing from the padelectrode 111 into the semiconductor substrate 110 may reach the backsurface of the semiconductor substrate 110 and cause dielectricbreakdown. Then heat generated at the time may cause the semiconductorsubstrate 110 to crack, thereby breaking the igniter.

SUMMARY OF THE INVENTION

The invention provides a semiconductor die that includes a semiconductorsubstrate of a first conductive type, a first semiconductor layer of asecond conductive type disposed on the front surface of thesemiconductor substrate, a second semiconductor layer of the firstconductive type disposed on the first semiconductor layer, and a firstelectrode and a second electrode that are in contact with the secondsemiconductor layer so that a current runs between the two electrodesthrough the second semiconductor layer under a normal input currentcondition. The die also includes a metal film disposed on the backsurface of the semiconductor substrate so as to be in a direct physicalcontact with the back surface. The semiconductor die is bonded to anisland of a lead frame by a conductive paste so that the metal film isattached to the island through the conductive paste, and thesemiconductor substrate and the first and second semiconductor layersare configured to operate as a parasitic bipolar transistor so as torelease an input current to the island under a surge event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a semiconductor device of anembodiment of the invention and the peripheral circuit.

FIG. 2 is a plan view showing a schematic structure of the semiconductordevice of the embodiment of the invention.

FIG. 3 is a cross-sectional view showing the semiconductor device of theembodiment of the invention.

FIG. 4 is a cross-sectional view showing a layering structure of a metalthin film of the semiconductor device in FIG. 3.

FIG. 5 is a cross-sectional view showing a semiconductor device of acomparison example.

FIG. 6 is a cross-sectional view showing a conventional semiconductordevice.

DETAILED DESCRIPTION OF THE INVENTION

A semiconductor device of an embodiment of the invention will bedescribed referring to figures. FIG. 1 is a circuit diagram showing aschematic structure of this semiconductor device and the peripheralcircuit. This semiconductor device is a semiconductor device that tendsto be applied with a surge (e.g. a pulse-shaped overvoltage having ahigh leading edge of an amplitude), for example, an igniter 1 forvehicles. FIG. 2 is a plan view showing a schematic structure of theigniter 1 in FIG. 1.

As shown in FIG. 1, the igniter 1 has a function of controlling theignition of an ignition plug 2 of an engine on a vehicle, and includes aplurality of semiconductor dies, for example, an IC die 10A formed as acontrol circuit, an IGBT die 10B as a switching element, and so on. Theigniter 1 controls the switching operation of the IGBT die 10B by the ICdie 10A, and causes self-induction by blocking a current flowing from apower supply into a primary coil 3A of an ignition coil by the IGBT die10B to generate a high voltage at a secondary coil 3B of the ignitioncoil. Ignition occurs when this high voltage is applied to the ignitionplug 2.

In the igniter 1 mounted on a vehicle, a large surge caused by noisegenerated at other device such as a motor mounted on the vehicle tendsto be applied to the IC die 10A through a power terminal T1 and wires.In particular, a surge tends to occur when the engine of a vehicle isstopped.

The IC die 10A and the IGBT die 10B forming the igniter 1 arerespectively bonded to islands 51, 61 of lead frames 50, 60 made ofmetal such as copper, as shown in FIG. 2, for example, and sealed with aresin (not shown) according to need. In the figure, the IC die 10A andthe IGBT die 10B are connected to lead terminals 72 through bondingwires 71. One of the plurality of lead terminals 72 is formed as thepower terminal T1 connected to the power supply, for example.Furthermore, a die capacitor 4 is disposed between the two islands 51,61, connecting these.

Hereafter, the IC die 10A bonded to the island 51 of the lead frame 50will be described referring to figures. FIG. 3 is a cross-sectional viewshowing the IC die 10A in the igniter 1. In FIG. 3, a region where aprotective resistance layer is formed in the igniter 1 and the peripheryare simply shown, and the other components, for example, a region wherea transistor is formed is omitted in the figure. Furthermore, FIG. 4 isan enlarged cross-sectional view showing a layering structure of a metalthin film 30 in FIG. 3.

As shown in FIG. 3, the IC die 10A is formed by a semiconductorsubstrate 10 as a P type silicon substrate. In the front surface of thesemiconductor substrate 10, an N type embedded layer 11 is disposed andan N type epitaxial layer 12 is disposed thereon. A P type semiconductorlayer 13 is disposed in part of the front surface of the epitaxial layer12. In the example shown in the figure, both the ends of the embeddedlayer 11 are extended to the front surface of the IC die 10A. Theepitaxial layer 12 exists on the outside of both the ends of theembedded layer 11, and a P type element isolation layer 14 connected tothe front surface of the semiconductor substrate 10 is disposed on theoutside of the epitaxial layer 12.

The semiconductor layer 13 of the embodiment is used as the protectiveresistance layer of the igniter 1, which moderates a surge current by apredetermined resistance value R. In this case, one end of the frontsurface of the semiconductor layer 13 is connected to a pad electrode 16through an opening of the insulation film 15. The pad electrode 16 isconnected to the power supply through the bonding wire 71 connected tothe power terminal T1 in FIG. 1. Another end of the semiconductor layer13 is connected to a wire 17 through an opening of the insulation film15. The wire 17 is connected to other element (not shown). The elementisolation layer 14 is connected to a grounded wire 18 through an openingof the insulation film 15.

The metal thin film 30 is disposed on the back surface side of the ICdie 10A, being in direct contact with the back surface of thesemiconductor substrate 10 and covering the back surface. It ispreferable that the metal thin film 30 covers the whole back surface ofthe semiconductor substrate 10. A conductive paste 40 made of conductiveparticles and a resin is disposed between the metal thin film 30 and theisland 51. The conductive paste 40 is in direct contact with the metalthin film 30 and with the grounded island 51.

It is preferable that the conductive paste 40 is a silver pastecontaining silver particles as conductive particles. Die-bonding using asilver paste has an advantage that the process temperature for bondingand the manufacturing cost are lower than those of die-bonding usingother material, for example, a lead-free solder.

In a process of manufacturing an IC die, an oxide film, namely, anatural oxide film (not shown) is formed on the back surface of thesemiconductor substrate 10, which is naturally formed by the oxidationof the semiconductor substrate 10 as a silicon substrate after backgrinding, for example. Immediately after this natural oxide film isremoved by, for example, a plasma etching treatment, the metal thin film30 is formed on the back surface of the semiconductor substrate 10 by avapor deposition method. By this, the back surface of the semiconductorsubstrate 10 and the metal thin film 30 are in direct contact with eachother without through the natural oxide film, and realizes a stablecurrent flow from the semiconductor substrate 10 to the island 51.

As shown in FIG. 4, the metal thin film 30 is formed by sequentiallylayering an aluminum layer 31 formed being in direct contact with theback surface of the semiconductor substrate 10, a chromium layer 32, acopper layer 33 and a gold layer 34 on the back surface side of the ICdie 10A, i.e., on the back surface side of the semiconductor substrate10. The aluminum layer 31 enhances the contact with the semiconductorsubstrate 10, the chromium layer 32 prevents inter-reaction between thealuminum layer 31 and the copper layer 33, the copper layer 33 decreasesthe total electric resistance of the metal thin film 30, and the goldlayer 34 prevents the oxidation of the surface of the copper layer 33.The gold layer 34 is in direct contact with the conductive paste 40containing conductive particles 41 (preferably, silver particles) and aresin 42. This metal thin film 30 has a thickness of, for example, about0.5 to 1.5 μm in total.

In this IC die 10A, as apparent from the cross-sectional structure inFIG. 3, a PNP bipolar transistor that uses the P type semiconductorsubstrate 10 as a collector, the N type embedded layer 11 and epitaxiallayer 12 as a base, and the P type semiconductor layer 13 as an emitter,i.e., a parasitic transistor Trp is formed.

When the potential of a surge applied to the semiconductor layer 13through the bonding wire 71 and the pad electrode 16 is high enough tobreak down the parasitic transistor Trp, the surge current flows intothe P type semiconductor substrate 10 from the semiconductor layer 13through the parasitic transistor Trp. This surge current flows into thegrounded wire 18 through the P type element isolation layer 14 (i.e. afirst path), and also flows into the grounded island 51 from the backsurface of the semiconductor substrate 10 through the metal thin film 30(i.e. a second path).

When the parasitic transistor Trp does not break down even when a surgeis applied to the semiconductor layer 13, the surge current does notflow into the semiconductor substrate 10 and is moderated correspondingto the resistance value R of the semiconductor layer 13 as a protectiveresistance layer and flows in the wire 17.

Here, a case where the metal thin film 30 is not formed on the backsurface of the semiconductor substrate 10 as a comparison examplerelative to the structure of the IC die 10A described above will beconsidered. In this case, as shown in a cross-sectional view in FIG. 5,a natural oxide film 110F remains on the back surface of thesemiconductor substrate 10, which is formed by the oxidation of thesemiconductor substrate 10 as a silicon substrate after theback-grinding of the semiconductor substrate 10, for example. Then, withthis state, the conductive paste 40 is disposed between the naturaloxide film 110F and the island 51.

This natural oxide film 110F does not have a constant thickness and aconstant state on the back surface of the semiconductor substrate 10,and partially has a portion where the dielectric strength is low toeasily cause dielectric breakdown, for example, a thin portion 110T.Therefore, when a surge current flows into the semiconductor substrate10 through the parasitic transistor Trp, in the portion 110T of thenatural oxide film 110F, which easily causes dielectric breakdown,dielectric breakdown occurs due to the surge current with high currentdensity. With heat generated at the time, a crack 10CL occurs in thesemiconductor substrate 10 and the IC die 10A is damaged. This crack10CL may extend from the semiconductor substrate 10 to the semiconductorlayer 13 that is superposed on the pad electrode 16, and may furtherextend to the interface of the pad electrode 16 and the semiconductorlayer 13 to penetrate the IC die 10A.

On the contrary, in the IC die 10A of the embodiment, a surge currentflowing into the P type semiconductor substrate 10 through the parasitictransistor Trp flows in the first path directed to the P type elementisolation layer 14 and the grounded wire 18 and further flows in thesecond path directed to the grounded island from the back surface of thesemiconductor substrate 10 through the metal thin film 30. Therefore,the crack 10CL does not occur in the semiconductor substrate 10 and soon by the dielectric breakdown of the natural oxide film 110F,preventing the IC die 10A from being damaged. In particular, when themetal thin film 30 is formed over the whole back surface of thesemiconductor substrate 10, the second path is widened to realize astable surge current flow from the semiconductor substrate 10 to theisland 51.

The invention is not limited to the embodiment described above, andmodifications are possible within the scope of the invention.

For example, in the embodiment described above, the description is givenfor the case where the semiconductor layer 13 of the IC die 10A is theprotective resistance layer of the igniter 1. However, the invention isnot limited to this and also applicable to a region where other element,for example, a transistor is formed. In this case, the transistor isformed by using at least the P type semiconductor substrate 10, the Ntype embedded layer 11, the epitaxial layer 12 and the P typesemiconductor layer 13.

Furthermore, the semiconductor device of the embodiment described aboveis referred to as the igniter 1 for vehicles, the invention is notlimited to this and also applicable to other semiconductor device forvehicles as long as a surge tends to be applied thereto. Accordingly,the resistance to a surge in a semiconductor device is enhanced.

What is claimed is:
 1. A semiconductor die comprising: a semiconductorsubstrate of a first conductive type; a first semiconductor layer of asecond conductive type disposed on a front surface of the semiconductorsubstrate; a second semiconductor layer of the first conductive typedisposed on the first semiconductor layer; a first electrode and asecond electrode that are in contact with the second semiconductor layerso that a current runs between the two electrodes through the secondsemiconductor layer under a normal input current condition; and a metalfilm disposed on a back surface of the semiconductor substrate so as tobe in a direct physical contact with the back surface, wherein thesemiconductor die is bonded to an island of a lead frame by a conductivepaste so that the metal film is attached to the island through theconductive paste, and the semiconductor substrate and the first andsecond semiconductor layers are configured to operate as a parasiticbipolar transistor so as to release an input current to the island undera surge event.
 2. The semiconductor die of claim 1, wherein the metalthin film comprises an aluminum layer, a chromium layer, a copper layerand a gold layer that are sequentially layered on the back surface sideof the semiconductor substrate.
 3. The semiconductor die of claim 1,wherein the conductive paste is a silver paste comprising silverparticles.
 4. The semiconductor die of claim 1, wherein the firstelectrode is connected to a power supply.
 5. The semiconductor die ofclaim 1, wherein the semiconductor die is a semiconductor device for avehicle.
 6. The semiconductor die of claim 1, wherein the semiconductordie is a semiconductor device for an igniter.